The present invention relates generally to bipolar electrolytic cells for the production of metal, and particularly to a cell in which the accumulation of molten metal on the cathode of the cell will not short the cathode to the electrode immediately adjacent the cathode when the cell is operated in a standby, heating mode, as discussed below.
In U.S. Pat. No. 4,021,317 to Knapp et al, there is disclosed a method and apparatus for preheating or maintaining the heat existing in an electrolytic cell using the inherent resistance of the terminal cathode and/or anode of the cell. The patent discloses, in addition, a switching device for changing the cell from a mode in which metal is produced to the mode in which the cell can be heated using the resistance of its anode and cathode. Ordinarily, it is expected that the cell in a preheat or standby heat condition would not produce metal, as the conditions for producing metal are supposedly not prevalent, i.e., there is no electrical potential applied across the anode and cathode surfaces of the bipolar electrodes, which potential is necessary for decomposing a metallic compound in a solvent bath.
However, it has been found that when the anode and cathode are connected in electrical series with the power supply of the cell, and the cell contains an appropriate electrolyte, metal is produced on the cathode because of the voltage conditions that prevail in the cell, i.e., a potential difference is established between portions of the anode and cathode such that the electrolyte decomposes and metal is produced on at least that portion of the cathode surface exposed to the potential (voltage) difference.
It has further been found, for example, that in a quiescent electrolyte of aluminum chloride a drop of molten aluminum will grow to a height of about 1.59 centimeters (0.625 inches) when deposited on a flat, horizontal, graphite surface immersed in the electrolyte before the drop of aluminum begins to spread. Other metals behave in a similar manner, the maximum height of the metal on a non-wetting surface being dependent upon the difference between the surface tensions and densities of the metal and the electrolyte.
Ordinarily, in the operation of a cell in which the metal produced in bipolar compartments is swept from the compartments by gas generated in the compartments, such as shown and described in U.S. Pat. No. 3,822,195 to Dell et al, for example, the buildup of metal on the bipolars is not a problem. Under standby heating conditions similar to those described by Knapp et al in U.S. Pat. No. 4,021,317, it has been found, however, that little or no gas is generated in the interelectrode spaces near the terminal cathode; therefore, the sweeping of metal from these surfaces is inhibited. For this reason it is incumbent upon the designer of electrolytic cells, using superposed bipolar electrodes for the production of metal, to space the cathode and the next adjacent electrode a distance apart that is greater than the height at which the molten metal produced in the space will accumulate and build on the cathode before the metal begins to spread out and run off the cathode.
The adequacy of the dimension of this space is particularly important when a cell is placed in a mode for heating, using the resistance of the anode and cathode to the flow of electrical current as the source of heat. In such a mode, there is no generation of gas in the bipolar compartments such that there is no sweeping action across the surfaces of any of the electrodes to remove any metal on such surfaces. Any metal present on the cathode when the cell is changed to a heating mode is of course not swept away. The difference in potential between the anode and cathode when the cell is placed in a heating mode begins to produce metal on a portion of the cathode, as explained earlier. This metal is deposited upon any metal already existing on the cathode when the cell is changed to the heating mode such that the cathode can become shorted to the bipolar electrode immediately adjacent the cathode in a relatively short period of time, this time period depending upon the design, operating parameters and contents of the cell. When this occurs the bipolar and its compartment with the cathode are removed from operation which affects directly the efficiency and overall operation of the cell, as the cell is designed to operate, when producing metal, with a predetermined number of bipolars and compartments that share the potential imposed across the electrolytic bath. In this manner, a predetermined, proper potential is provided across each compartment for decomposing the electrolyte. In addition, when the cathode and the next adjacent bipolar short, this bipolar becomes the cathode for the cell and begins to accumulate metal itself. If the cell remains in standby condition a sufficient length of time, the metal collected on this bipolar will reach the next bipolar and electrically short the two together.
In U.S. Pat. No. 4,025,413 to Nightingale et al, the problem of slime collecting in the bottom of a tank for refining copper is discussed, the slime shorting adjacent, vertically oriented electrodes together. To cure the problem, the practice in that art had been to raise the electrodes from the bottom tank surface. This, however, as explained in the text of the patent, created other problems, the disclosure of the patent being directed to the use of vertically extending, insulating plates of a lattice work located below the electrodes to reduce current flow between the lower edges of the electrodes during the refining process.